A conventional image forming apparatus will be described below. FIG. 16 shows the structure of a print position adjustment mechanism in a conventional image forming apparatus. FIG. 16 shows a photosensitive drum 31, a laser device 202 which forms a latent image on the photosensitive drum 31, a registration clutch (to be also referred to as registration rollers hereinafter) 203 which determines the paper feed timing, a paper sensor 1204 for detecting a paper sheet to be conveyed, a deviation amount detection sensor 1205 which detects the deviation amount of the widthwise end in a direction (to be also referred to as a widthwise direction hereinafter) perpendicular to the paper feed direction, an output paper sheet 107, and a paper convey path 205.
In the print position adjustment mechanism of the conventional image forming apparatus with the above arrangement, a control circuit (not shown) detects the deviation amount of a paper sheet in its widthwise direction using the deviation amount detection sensor 1205, and detects the paper position in the paper feed direction using the paper sensor 1204. Furthermore, the control circuit adjusts the transfer timing of image data to a laser control circuit (not shown) that drives the laser device 202, and the paper feed timing of the registration clutch 203 on the basis of these pieces of acquired information.
Furthermore, the control circuit sets the image forming start position (laser irradiation start position) of the laser device 202, and checks any skew of a paper sheet on the basis of at least two widthwise end positions of the paper sheet detected by the deviation amount detection sensor 1205 to make error display and the like (e.g., Japanese Patent Laid-Open No. 9-219776).
However, in the conventional image forming apparatus, the image position precision in the paper feed (convey) direction is dominantly determined by the coupling time of the registration clutch. Especially, when a high-speed print process is done, the image position precision deteriorates in proportion to the print speed due to the coupling time of the registration clutch.
When a high-speed print process is done, the image position precision also deteriorates due to some detection error of the paper feed timing by the sensor, mechanical attachment errors and durability of components, and the like.
It is, therefore, an object of the present invention to provide an image forming apparatus and image formation control method, which can detect the paper feed timing with high precision, can eliminate any drop of the image position precision due to mechanical attachment errors and durability of components, and can precisely adjust the image position all the time.